Code division multiple access (CDMA) is a technique used in wireless communications systems, such as personal communication service (PCS) systems which provide integrated wireless data services, for permitting simultaneous access by multiple wireless devices to a single uplink voice channel of a base station. In CDMA, a digital carrier signal is modified by a code uniquely identifying a particular wireless device, thus effectively spreading the digital signal over a greater bandwidth. For example, a 10 kHz signal may be modulated with a code that effectively creates a 100 kHz-spread signal. This is also true in the downlink direction.
The spread signal modulates a carrier frequency signal that is in the range of the uplink frequency channel. The modulated carrier is then transmitted from the wireless device to the base station. The base station receives a composite spread signal of all the modulated carrier frequency signals from all wireless devices transmitting at that point in time. The base station demodulates the composite spread signal and then despreads the demodulated signal using the same unique code used at a particular wireless device for spreading. The despread signal approximates the pre-spreading data signal for the particular wireless device. Again, this is true in the downlink direction.
Within a sector of a base station, the population of PCS users can be split into fixed and mobile wireless users. A fixed user is a user that stays in place or moves at a relatively low rate of speed such as when walking during a call. A mobile user is a user that moves at a relatively high rate of speed such as when traveling in a vehicle during a call. All users are located within a sector of the base station for wireless communication with the base station.
Typically, each user/subscriber device in the sector is allocated the same amount of power to communicate with the base station. The measure of power allocated to each user is defined as the ratio of energy per bit to the noise power spectral density (Eb/No). The Eb/No level for each user is measured at the base station. The base station then signals the subscriber device to adjust transmit power through a control loop so that the receive signal strength at the base station from all transmitting subscriber devices is approximately the same. System traffic capacity is limited to a finite amount of bandwidth. Thus, utilization of this capacity must be as efficient as possible.
Adding to the problem of finite system capacity, known systems that provide integrated data services require dedication of a single traffic channel for each user. In other words, the traffic channel is not shared. However, with such an arrangement, efficiency in system capacity utilization is compromised. Consequently, a need exists for a system and method which can assign data traffic channels to multiple users in a wireless communication system which improves efficiency of system utilization.